A hiker stands on one end of a canyon and shouts to hear his echo. If it takes 3 secounds for him to hear his echo what is the with of the canyon?

The sound waves have to travel across the canyon and back again for him to hear the echo, so the distance traveled in 3 seconds is two times the width of the canyon. If you know the speed of sound you multiply that by 3 seconds to get the distance traveled across the canyon and back, and then divide by two to get the width of the canyon.